U.S. Pat. No. 4,631,767 to Carr et al. describes an air flotation mattress for the treatment of bedridden patients. The patent teaches that conventional mattresses obstruct both the flow of air to the skin of a patient and the dissipation of heat away from the areas of the patient's skin in contact with the mattress. Moreover, decubitus ulcers, commonly referred to as bed sores, develop as a result of the combination of heat, moisture, pressure and shear forces on the body areas in contact with the mattress. The occurrence of decubitus ulcers is reduced by use of a highly deformable mattress, such as a water-containing mattress, but high local pressure still exists in the deformed tissue of the patient to compress blood vessels and to cause tissue damage.
Improvements in the treatment of bedridden persons have been provided by use of air mattresses as described in Carr et al. An air mattress allows continuous air flow through the mattress to provide ventilation for of the patient's skin. The ventilation facilitates moisture dissipation and acts as an insulator to avoid skin abrasion. Since shearing force, heat, moisture and abrasion are leading causes of decubitus ulcers, the air mattress significantly reduces the risk of formation of tissue damage. Other such air mattresses are described in U.S. Pat. Nos. 3,740,777 to Dee, 4,279,044 to Douglas, 4,485,505 to Paul, and 4,766,629 to Schueler.
Another device for increasing the comfort of a bedridden person is described in U.S. Pat. No. 4,799,276 to Kadish. The patent describes a bed having a matrix of supports in the form of free-floating, pressurized, air-supported pistons. The pressure exerted by a patient's body sections against individual support systems is constantly measured. Valves control the discharge of the air from the piston chambers of the individual support pistons, so that after a preset time and level of pressure prevailing within a given piston chamber, a fixed amount of air is discharged. The discharge of air causes the support system to move downwardly to allow a change in the position of the patient. At the end of a cycle, all of the pistons are brought back to their initial high level.
One problem with the air-release mattresses described above, is that the constant air flow requires a high volume of air to be channeled into the mattress. Thus, the blower must be a high-volume pump which is typically more expensive and more noise-generating than a pump which is not required to move a high volume of air. The matrix of support systems which are individually valved to automatically reposition a patient after a preset period of time, as taught by Kadish, overcomes this requirement of a high-volume pump. However, Kadish requires a microprocessor controller or the like to constantly monitor the individual support systems. The device is not an inexpensive one.
U.S. Pat. No. 3,942,202 to Chevrolet teaches an air support mattress which reduces the volume of air necessary for providing a mattress having an array of vents to direct air at a patient. Each vent of the Chevrolet mattress includes a pair of lips which meet tangentially to normally block air flow through the vent. When a person lies on the mattress, the weight of the person pulls the lips apart to allow a stream of air to flow through the vents and exert a supporting force on the body part depressed on the mattress. That is, an unoccupied mattress does not include a flow of air through the upper wall of the mattress, but upon deformation of the mattress, air flow is initiated. While the force applied to the mattress is a vertical force provided by the weight of the person, the separation of the lips requires a horizontal component of force. Because the lips are separated horizontally and because the mattress is made of material which readily deforms, individual vents are opened even when located at a distance from the patient that guarantees that no air released from the vent will come in contact with the patient. Depending on the material used in the manufacture of the mattress, it is possible that each vent of the mattress will be opened by the weight of a patient, thereby offering no improvement over the high-volume requirements of the prior art.
It is an object of the present invention to provide a body support device having an upward release of gas wherein the release is selectively localized. It is a further object to provide such a device which consumes less power and produces less noise than the prior art without an increase in cost of the device.